1. Field of the Invention
The present invention relates to a synthetic resin composition which is used in OA instruments, AV instruments, cameras, telephones, electric shavers, wheel caps for automobiles, cosmetic vessels, and the like, and more particularly, it relates to a synthetic resin composition for producing a molded article having a metallic appearance like the brilliant Milky Way.
2. Description of the Prior Art
A conventional method is known for producing a molded article having a metallic appearance like the brilliant Milky Way by mixing a thermoplastic resin and a metal powder, and melt molding the mixture. However, where the thermoplastic resin containing the metal particles is injection-molded, a poor appearances are liable to occur such as a weld line, which is formed at the mating portion of the resins, a flow pattern of the resin or the like (hereinafter those are referred to as "weld appearance" in combination), and its usage was largely restricted. Various improved methods have been proposed against this problem. For example, JP-A-51-63847 (the term "JP-A" used herein means an unexamined published Japanese patent application) discloses a synthetic resin composition comprising 100 parts by weight of a synthetic resin and from 0.3 to 10 parts by weight of an aluminum pigment having an average particle diameter of 30 .mu.m or less and an average shape ratio (thickness/particle size) of from 1/20 to 1. Further, JP-A-61-159453 discloses a resin molded article comprising a thermoplastic resin, and particles having a surface luster having an average particle diameter of from 35 .mu.m to 1 mm and an average shape ratio of from 1/8 to 1 in an amount of from 0.1 to 15 parts by volume per 100 parts by volume of the sum of the thermoplastic resin and the particles having the surface luster. Those prior art intend to improve the weld appearance by using a metal powder having a large average shape ratio (thickness/particle diameter) and a shape relatively close to a spherical shape to decrease the directionality of the particles. In addition, JP-A-61-49817 discloses an improved weld appearance obtained by adjusting the particle diameter and the amount of metal particles to be added such that a composition comprises 100 parts by volume of a thermoplastic resin and from 0.1 to 20 parts by volume of the metal particles having a maximum outer diameter of from 10 .mu.m to 1 mm and an optional shape, and the metal particles have the relationship of D.gtoreq.H between an average interval D and a, weld width H.
However, the above-described conventional method using the metal particles involves a pulverization in the metal powder formation step, and therefore it cannot prevent contamination of extremely fine particles. If such extremely fine particles of a metal powder are contaminated, the extremely fine particles act just like a black pigment, whereby a blackish weld line or flow pattern are developed, and as a result, an excellent weld appearance property cannot be attained. This is just a reason why the weld appearance deteriorates as the amount of the metal powder to be added is increased. In addition, the fine particles adversely affect to darken the molded article, and if a white pigment such as titanium white is added to improve this defect, a metallic feel is decreased due to a hiding affect.
In order to minimize the amount of such metal fine particles to be contaminated, JP-A-61-241333 discloses a metallic pigment for incorporating into a synthetic resin, comprising metal fine particles having an average shape ratio (thickness/particle diameter) of from 1/25 to 1, a passing percentage of a 500 mesh Tyler standard screen of 30% or less and preferably 20% or less, and a passing percentage of a 200 mesh Tyler standard screen of 90% or more and preferably 95% or more, and the JP-A-61-241333 proposes to limit the average shape ratio of the metal powder as well as limiting sieves sizes and also to use particles mainly having a particle diameter of from 25 to 74 .mu.m as the metal particles,
However, even if this technique is used, the particle diameter distribution is wide and the contamination of metal fine particles is still unavoidable Y, the above-described defect is left unsolved. In particular, in a polyester type resin such as a filler-reinforced polyethylene terephthalate resin (hereinafter referred to as "GF-PET resin"), or an alloy resin of a polycarbonate resin (hereinafter referred to as "PC resin") end a polyethylene terephthalate resin (hereinafter referred to as "PET resin"), such problems markedly arise that a hue of a base resin becomes blackish or a weld appearance deteriorates. This is assumed, in the case where such polyester type resins are used, to be caused since a high shear must be exerted on the resins at a melt pelletization with a twin screw extruder or the like, the metal particles are further pulverized, thereby generating fine particles. In fact, when a molded article was observed with an electron microscope, extremely fine particles were observed.
In view of the above-described problems involved in the prior art, one object of the present invention is to provide a synthetic resin composition which gives an excellent weld appearance property and a brilliant metallic feel, which have not yet been attained by the prior art, to the high-grade molded products which have a complicated shape and are liable to cause a weld line or a flow pattern, such as chassis for OA instruments, chassis for AV instruments, exterior covers for cameras, wheel caps for automobiles, and the like, and which can prevent a base color from being blacked even in the case where polyester type resins are used.
Another object of the present invention is to provide a metallic pigment for incorporating into a synthetic resin to be used in the synthetic resin composition.